10.8 Peripheral Nerve Monitoring

Question 1

How can neuromuscular function be monitored?

Answer 1

Clinical
* Grip strength

• Ability to sustain head lift for at least 5 seconds
• Ability to produce vital capacity of at least 10 mL/kg

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Neuromuscular stimulation equipment

• Peripheral nerve stimulator
• Mechanomyography:
  uses force transducer to quantitatively measure contractile response
• Acceleromyography:
  measures movement of joints caused by muscle movement
• Electromyography:
  measures electrical activity associated with action
  potential propagation in a muscle cell (research use

Question 2

What are the important characteristics of the peripheral nerve stimulator?

Answer 2

Portable, battery-powered, and easy to use

Able to deliver different impulses:

• Supramaximal current output of 50–60 mA at
  all frequencies to ensure all
  nerve fibres are depolarised
• Monophasic square waveform
• Single twitch at 0.1 Hz
• Train of four (TOF) at 2 Hz
• Tetanic stimulation at 50 Hz

Question 3

What is supramaximal stimulus?

Answer 3

If a nerve is stimulated with sufficient intensity,

all fibres supplied by the nerve contract
and a maximum response is triggered which depends on the
number of muscle fibres activated.

This stimulus should be truly maximal throughout the test period
to maintain accuracy;
hence the electrical current applied is at least
20% to 25% above that necessary for a maximal response.

Question 4

What do you mean by train of four (TOF)?

Answer 4

• The pattern involves stimulating the ulnar nerve
  with a TOF supramaximal twitch stimuli.
• Four stimuli are given at 0.5 s intervals,
  at frequency of 2 Hz.
• TOF is more sensitive than single twitches
  in monitoring neuromuscular blockade.
• Observer can compare T1 (first twitch of the TOF) to T0 (control).
• TOF ratio can be calculated by comparing T4 twitch height to T1.
• The extent of block can be deduced from number of TOF counts:

Question 5

TOF count and extent of block

Answer 5

TOF count Extent of block

1–2–3 T4 lost 75%
1–2 T3–T4 lost 80%
1 T2–T3–T4 lost 90%
0 T1–T2–T3–T4 lost 100%

Question 6

explain the observation following non-depolarising and depolarising neuromuscular blocking agents.

Answer 6

Non-depolarising neuromuscular blocking agents (NDMB):

repetitive stimulation (ToF or tetanus) is associated with fade (
reduction in amplitude of evoked responses with T4
affected first, then T3, followed by T2, then finally T1)
and post-tetanic facilitation.

Depolarising neuromuscular blocking agents (DMB):

no fade or post tetanic facilitation observed.

Repeated dose of suxamethonium can give
characteristics of NDMB—phase II block).

Question 7

What counts indicate surgical relaxation and safe extubation

Answer 7

ToF ratio of 0.15 – 0.25: indicates adequate surgical relaxation
ToF ratio of > 0.9: essential for safe extubation and recovery post surgery

Question 8

What is double burst stimulation?

Answer 8

• Used when a profound block is present.
• Two bursts of tetanic stimulation at 50 Hz,
  separated by 750 msec are given.
• The duration of each square wave impulse in the burst is 0.2 msec.
• DBS was developed with the specific aim of allowing manual (tactile)
  detection of small amounts of residual blockade under clinical conditions.

Question 9

Which nerves are normally used for stimulation?

Answer 9

• Ulnar nerve (adductor pollicis—adducts thumb)
• Zygomatic branch of facial nerve—orbicularis oculi muscle
• Peroneal nerve—dorsiflexion of foot
• Posterior tibial nerve—plantar flexion of big toe

The diaphragm is the most resistant (but with shorter onset times) of
all muscles to both depolarising and non-depolarising relaxants requiring
1.5 to 2 times as much drug as the adductor pollicis muscle for an identical
degree of blockade.

Question 10

What are the differences between phase i and phase ii blocks?

Answer 10